Perhaps of earliest boat designs were generally limited to so-called displacement hulls, which had certain inherent speed limitations. It was probably sometime later that boat makers discovered the planing hull principle, which permitted much greater speeds at some sacrifice of stability. Multihull designs were created to combine at least some of the stability advantages of displacement hulls with the speed advantages of planing hulls. It is well known that, through sail power alone, a multihull sailing craft can achieve at least a semi-planing condition which overcomes some of the inherent speed limitations of a displacement hull while still retaining some semblance of stability.
An important stabilization factor in all existing multihull designs (e.g. trimarans and catamarans) is the static displacement of water by the leeward hull, creating a righting force on a moment arm of one-half of the total beam, i.e. the beam of the total multihull craft, including both outboard hulls and the center hull, if any. The static displacement force is the Archimedes force of the weight of the displaced water acting perpendicularly to the water surface. As is known in the art, gravitational forces can also be used to counteract the heeling moment produced by the wind on the sails; in smaller boat designs such as day sailing catamarans, the shifting of passenger weight is a simple means for applying such gravitational forces.
Nevertheless, users of multihull sailing craft do encounter stability problems. For example, it is possible for the heeling moment to equal or exceed the righting moment (including the moment of gravitational forces about the leeward ama), in which case capsizing becomes possible. For most conditions, the critical factor acting against capsizing of larger multihull craft is a force F acting generally parallel to the mast, i.e. generally perpendicular to the horizontal plane of the vessel's deck. An important function of the ama (sponson, pontoon, outboard hull, etc.) is to produce this force, which acts upward at the approximate longitudinal and transverse centers of the ama. Thus the force F produces a moment FB where B is one-half the transverse distance between the longitudinal axes of the amas. For convenience of discussion, one can disregard gravitational forces and concentrate upon the FB moment of force, particularly for off-shore cruising yachts, where stability enhancement is a primary concern and gravitational forces come into play only for nonsubmersible amas under extreme wind conditions.
Following these principles, many multihull sailing craft designers have specified extremely lightweight construction of the craft and have employed slim, low buoyancy amas and large crossbeams to produce the desired righting moments. These restrictions have produced vessels that, because of their large beam widths (i.e. the beam for the total vessel including all hulls) are under great stress. Yet, because of concern with the weight of the craft, these vessels are built with minimum structure. Furthermore, the slim, low buoyancy amas may become easily submerged, at which time they add nothing to the righting moment.
Another approach would be to utilize amas with greater buoyancy and wider beams (i.e. the beam of each individual ama would be relatively wide as compared to the aforementioned slim, low buoyancy amas). Such an approach typically leads to much greater drag coefficients for the blunter (greater cross-section) higher displacement amas.
The subject of drag (e.g. hydrodynamic drag) also bears a relationship to problems of stability encountered by designers and users of multihull sailing craft. Theoretically, drag (water resistance and/or air resistance) is proportional to the square of the velocity of the boat through the water. Although, as a practical matter, it is virtually impossible to calculate the total drag on a sailboat through theoretical considerations alone, it appears to be entirely valid to assume an exponential relationship between fluid resistance and the velocity through the fluid, hence exponential decreases in velocity caused by increasing air and water resistance, particularly increasing water resistance. Thus, as can be produced by the quick immersion of a blunt high buoyancy ama, or if some other discontinuous, drastic increment in total drag is felt by the craft, the resulting exponential decrease in velocity through the water can have equally radical destabilizing effects such as pitchpoling (a forward tumbling action which results in a forward capsize).
In recent years, great strides have been made in improving the stability of single hulled powered boats. Among the innovations in this field has been the use of the so-called stepped-V hull design. Theoretically, the principles of designing single hull power boats should have virtually no applicability to the problems of multihull sailing craft. Power boats designed for high speed (planing or semi-planing speeds) typically have sufficient reserve power to restore the planing or semi-planing condition even in the event of a disproportionate increase in drag. Such reserve power is typically not available to sailing craft, even those provided with auxiliary power. (The typical auxiliary power unit is adequate for displacement hull speeds only.) Similarly, designers of power boats do not feel tied to the concept of low buoyancy hulls with high length/beam ratios. Quite the contrary: some very fast single hull power boats have a scow-like hull or some similar board beam design. A power boat design with a length/beam ratio less than 10:1 or even less than 8:1 would not be unusual.
The following list of references is believed to provide a representative sampling of pertinent disclosures drawn from the art of single hull or essentially single hull construction (i.e. including those hulls provided with one or more channels generally parallel to the keel):
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 529,065 Dodge November 13, 1894 1,010,376 Vonkeissler November 28, 1911 1,296,155 Bazaine March 4, 1919 3,226,739 Noe January 4, 1966 ______________________________________
The following references are believed to be representative of the pertinent state of the art regarding multihull craft:
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 209,414 Norcross October 29, 1878 2,781,735 Roberts et al February 19, 1957 3,665,885 Javes May 30, 1972 3,788,257 Miller January 29, 1974 3,796,175 Ford, Jr. March 12, 1974 3,871,316 Woodrich March 18, 1975 Re. 28,615 Keiper November 18, 1975 ______________________________________
The following additional references are believed to be of interest regarding the state of the art of pontoon and hull structure generally.
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 1,898,322 Strode February 21, 1933 3,299,847 Bertholf January 24, 1967 ______________________________________